Double/Multiple Stars Culpeper Astronomy Club June 18, 2018 Overview • Introductions • Some Telescope Basics • Double Stars • Constellations • Observing Session Aperture Magnification • Magnification for a specific telescope changes with the eyepiece used • Calculated by dividing the focal length (FL) of the telescope (usually marked on the optical tube) by the focal length (fl) of the eyepiece • Mag = FL / fl • For example: • My Stellarvue SV110ED with a 770mm focal length using a 10mm eyepiece produces 77x magnification (770/10=77x) • My 102mm Unitron with a 1500mm focal length using a 10mm eyepiece produces 150x magnification (1500/10=150x) • 30 inch Obsession, f4.5, about 3500mm focal length using 31mm eyepiece produces 112x (3500/31=112x) • Higher the magnification smaller the field of view (FOV) Resolving Power • Resolving power is the ability of an optical instrument to produce separate images of closely placed objects…a double star • In 1867, William Dawes determined the practical limit on resolving power for a telescope, known as the Dawes limit…the closest that two stars could be together and still be seen as two stars • The Dawes Limit is 4.56 seconds of arc, divided by the telescope aperture in inches; converted to metric (approx): PR = 120/DO) • For example, my SV110ED with 110mm aperture (120/110) has a resolving power of 1.09 arc seconds • My 102mm Unitron has a resolving power nearly the same at (120/102) 1.18 arc seconds • The 30 inch Obsession, theoretically, yields 0.16 arc seconds Binary/Multiple Stars – The Motivation • They can be observed with a large or small telescope • There are plenty of targets even for a small telescope • They are beautiful, like multicolored gems in the sky • They are a great challenge, some difficult to split • They can be observed even with LP and full moon around Categories of Binary/Multiple Stars - Optical • Two and more stars may just happen to be nearly along the same line of sight • With one star being much farther than the other • Viewed from Earth appear close together • Visually appear to be many of these • Alcor and Mizar are examples Categories of Binary/Multiple Stars - Physical • Telescopic – appear as one but separate through a telescope • Thousands that can be split with amateur telescopes • Astrometric - detected through changes in motion • Sirius B was detected in this manner • Eclipsing – detected through changes in brightness • Agol (the Demon Star in Perseus) is brightest example • Usually never see the companion star • Spectroscopic – detected through changes in spectral images over time • By definition can’t be split with telescope • Spectroscope distinguishes two suns traveling together in space Observing and Measuring Double Stars Binary/Multiple stars – Mizar and Alcor • Mizar: Middle star in the Big Dipper • Should be able to detect faint companion with naked eye – Alcor • Pair is commonly referred to as “Horse and Rider” • Mizar was first double star discovered and photographed • Secondary takes several thousand years to complete orbit • Distance: 80LY • Magnitudes: 2.0; 4.0 • Separation: 14” Binary/Multiple stars – Cor Caroli • Brightest star in Canes Venatici is Cor Caroli • Named for King Charles (one of them!) • The two stars are separated by 20 sec of arc, corresponding to a projected separation of 680 AU • Yellowish-white and lilac • Distance: 110LY • Magnitudes: 2.84; 5.60 • Separation: 19.6” Binary/Multiple stars – Izar • Izar (Epsilon Bootis) is easy to spot along the eastern side of the Bootes • The name Izar comes from the Arabic word for “veil.” • Difficult split because of magnitude difference • Gold and slate blue at about 200X • Distance: 203LY • Magnitude: 2.5; 4.6 • Separation: 2.8” Binary/Multiple stars – Gemini • Alpha Geminorium (Castor): • Both are viewed as yellow-white • Actually 6 stars gravitationally- bound making it a sextuplet system • Nice pair at 100x • Distance: 50LY • Magnitudes: 1.9; 2.9 • Separation: 2.2” • Kappa Geminorium • Distance: 43.4LY • Magnitudes: 3.7; 8.2 • Separation: 7.5” Binary/Multiple stars – Iota and Zeta Cancri • Iota Cancri: yellow-blue/white pair with 60,000 year period • Distance: 298LY • Magnitudes: 3.9; 6.8 • Separation: 30.6” • Zeta Cancri: Multiple star system; first yellow-white pair easily split; second not so; • Distance: 83.4LY • Magnitudes: 5.58; 5.99 • Separation: 5.06” (<1”) Binary/Multiple stars – Algieba • Brightest star (after Regulus) in the “Sickle” of Leo • Algieba, comes from the Arabic Al Jabbah (The Lion’s Mane). • A clean split requires a magnification of 100X or more • What makes Algeiba so visually striking is its rich golden-yellow hues • Distance: 130LY • Magnitudes: 2.4; 3.6 • Separation: 4.6” Binary/Multiple stars – Porrima • Virgo has the fine binary star Porrima, one of the few stars close enough to show detectable motion during our lifetime • Separation less than 1.0” in 2005 • The two stars in this system revolve about each other in 169 years • Distance: 39LY • Magnitudes: 3.48; 3.53 • Current Separation: 2.5” RESOURCES • Sky and Telescope website: http://www.skyandtelescope.com/ • Stellarium software application: www.Stellarium.org • Suggested books: HERCULES – “THE HERO” • The fifth largest constellation in the sky; has no first magnitude stars • Rasalgethi: (Alpha Herculis) • Multiple star system 360 LY Distant • Primary star is a red giant; 400X Sun • Orange-Green Pair • Hercules contains two Messier objects: • Messier 13 (M13, NGC 6205) – Globular Star Cluster • Messier 92 (M92, NGC 6341) - Globular Star Cluster CYGNUS – “THE SWAN” • Cygnus: Easy to find as it features a well-known asterism known as the Northern Cross • Double Stars • Albireo: Marks the head of the swan; also known as “the beak star”; yellow primary- blue companion • 61 Cygni: “Bessel’s Star”; composed of a pair of two dwarfs; first star to be measured • Deep Sky Objects • “Veil Nebula” - (NGC6990/2/5) • “Fireworks” Galaxy – NGC 6946 • M39 – Open Star Cluster LYRA – “THE LYRE” • Lyra represents the lyre of Orpheus, the musician and poet in Greek mythology • Epsilon Lyrae, The “Double Double” • Multiple star system about 162 LY distant • Apparent visual magnitude - 4.7 • Can be resolved into two binaries when observed through a telescope • Beta Lyrae, Eclipsing Double • Pair of stars in a very tight orbit • Light varies gently and continuously over its 12.9-day period • Deep Sky Objects: • M57 – Planetary Nebula • M56 – Globular Star Cluster METEOR SHOWERS UPCOMING EVENTS • Date: July 23, 2018 at 7-9 p.m. • Topic: Variable Stars • Mars Opposition: July 27 • Perseid Meteor Shower: August 11-14 • Comet 46P/Wirtanen: December 2018 COMET 46P/WIRTANEN • Small short-period comet - orbital period of 5.4 years • Discovered on Jan 17, 1948, by the American astronomer Carl A. Wirtanen • On 16 December 2018 the comet will pass within 7.2M miles of Earth • Estimated magnitude of 3 to 7.5 • Brightest predicted pass to date…and all future passes Stars • Binary/Multiple Stars • Numerous targets available • Can be Physical or visual Albireo • Provide great contrasting colored Iota Cas pairs • Can be challenging to split Double Stars Almach depending on scope size and atmospherics • Carbon Stars • Most are Red Giants • Red complexion comes from carbon in their atmosphere • Variable Stars R Leporis T Lyrae Garnet Star • Observed over time Carbon Stars.